O. Byrne, M.A. Collier, M.C. Ryan, and J.G. McCaffrey, Crystal field splitting on D↔S transitions of atomic, Low Temp. Phys. 36, (2010) [Fiz. Nizk. Temp. 36, 524-531, (2010)] DOI: https://doi.org/10.1063/1.3432260.